Frequency generator



April 2.8, 194,2.` 1 HAMMOND FREQUENCY GENERATOR Filed March 2, 1940 ||||||||||||||||I|||h|||||||||l 32 ff? Vex? for' a ure/zsm fno/2 d Patented Apr. 28, 1942 v UNITED STATI-:s PATENT vOFFICE FREQUENCY 'GENERATOR Laurens Hammond, Chicago, Ill..

Application March 2, 1940, Serial No. 321,835

(ci. irs- 363) 7 Claims.

My invention relates generally to electrical frequency generating means, and more particularly to electrostatic or varying capacity oscillation generators.

In endeavoring to generate electrical oscillations by means of a continuously varying capacitor which is polarized by applying a direct current potential thereto, it has been found that upon removal of the polarizing potential, the generator continues to supply an unwanted signal at a relatively low level. If the generator is to be used for any purpose which has as a requisite condition that the signal cease upon removal of the polarizing potential, as is the case when the generator is to be used as a signal source for an electrical musical instrument, such continued generation of the signal makes the useof the generator undesirable.

Apparently the continued sounding of the signal after the removal of the polarizing potential is due to a residual potential or polarization present in the generating system. However, this residual potential is not a constant potential for which compensation can be made by polarizing the generator in the opposite direction. In fact, not only is theie a variation in the polarizing potential which is found necessary to compensate for or cancel the residual polarization, but sometimes it is necessary to apply to the generator a positive potential while at other times a negative different metals used in the generator circuit.I

However, such theory does not appear fully to explain the presence of the residual potential since, if it were due to this contact potential effect, it would be expected that the direction of polarization wouldI be the same at all times.

extremely thin and irregular lm of oil which cannot be removed from the metallic parts by Washing in the usual solvents, or may be a film of gas adhering to the metallic surfaces. Another possibility is that the dielectric is formed by an extremely thin layer of a metallic oxide, or by particles of dust or other dielectric material adhering to the metallic surfaces.

I have discovered that this disadvantageous effect due to residual polarization of the generator may be substantially completely 4removed by applying locally tothe electrodes of the varying capacitor or electrostatic generator, a high frequency potential such as that supplied by a radio frequency oscillator.

It is therefore a. primary object of my invention to provide an improved varying capacitor or electrostatic frequency generator which will cease supplying a signal upon removal of the polarizing potential applied thereto.

A further object is to provide an improved frequency generating means in which the amplitude of the output may readily and consistently be varied from its maximum value to zero by controlling the value of the direct current polarizing potential applied thereto.

Other objects will appear from the following description, reference being had to the accompanying drawing in which:

Figure 1 is a diagrammatic view of a rotary type electrostatic generator or variable capacitance and associated circuits; and,

Figure 2 is a diagrammatic view of an electrostatic frequency generator of the vibrating reed type with associated circuits.

Referring to Figure 1, the generator comprises a disc or wheel I0 of metal having a plurality of salient `projections or teeth I2. The Wheel I0 is secured to a shaft I4 which is driven from a suitable source of power I6 by driving means such as pulleys I8 and 20 and belt 22.

An electrode 24 of metal is located closely adi jacent the path of movement of the ends of the It is therefore my theory, although I know l This dielectric may possibly be in the form of an teeth I2, the wheel I0 of the electrode 2l forming the plates of a condenser, the capacity of which is varied through a considerable range as the wheel I0 is rotated due to the change in distance between the electrode 24 and theperipheral `edge of the wheel I0. The electrode 24 is effectively maintained at ground potential through a circuit consisting of a radio frequency choke coil 26, and a grid resistor 28. The potential of the wheel, which is electrically connected to the shaft I4, may be made positive or negative with respect to ground, by connection to a potentiomerator by the choke 26, condenser I2 of Fig. 1.

reed 64 and its pickup electrodev 24.

eter resistance 30 connected across a direct current potential source 32. 'Ihe connection is made through a conductor 34, the secondary winding 36 of a transformer 26, and a contactor 40 which may be brought-into contact with the potentiometer resistance 30 at any 'point along the length thereof. An intermediate point 4l on the resistance 30 is` grounded through a conductor 43.

A source 42 of high frequency oscillations, which may be in the form of a radio frequency vacuum tube oscillator,` has its output connected to the primary 44 of the transformer 34, and thus, when in operation, may superimpose high frequency oscillations upon the polarizing potential applied between the wheel lll and electrode 24. The varying capacity between the wheel I0 and electrode 24 causes a voltage change which is impressed upon the grid 45 of an amplifying electron discharge device 46, the cathode 41 of which is connected to ground through a self-biasing resistor 48 by-passed by a condenser 50. 'Ihe plate 52 is connected -to a suitable source of plate current indicated as a terminal +250 V. through a load resistor 54. A blocking condenser 56 is connected between the plate 52 and one of two output terminals 58, the other terminal 58 being connected to ground.

theplate 52 and ground to remove the radio fre- .quency from the output'of the tube-.46. It will be understood that the frequency of the vgenerator 42,. if not at a radiofrequency, is preferably of sutllciently higher frequency than that generated Yby the wheel i0, that it-mayfbe substantially completely filtered from the output of the gen- 64 and condenser 62.

lIn Figure 2, thlmvention is illustrated dia.-

grammatically a's applied to a vibrating reed type generator comprising a suitably supported mecharged through the resistor 14, and resistors 14 and I6A respectively. The signal at the output terminal will thus have a. gradual attack.

Upon opening the switch 12, the polarizing potential willdrop gradually at a rate determined by the discharge of condensers 18 and 68 through the resistor 66. The resistors 14 and 16 are preferably of considerably lower value than the resistor4 66. Thus, the resistor 16 will not have an appreciable effect' upon the rate at which these condensers discharge. As a result of the gradual discharge of the condensers. the signal at the output terminals 56 will decay gradually. It will be apparent that the attack and decay rates may be predetermined by proper selection of the values for the resistors. 66, 14 and 16 and of the capacities of condensers 68, 18.

y the wheel III (or reed 44) and electrode 24 will be modulated due to the changes in the capacity between these elements caused by rotation of the salient projections l2 pastthe electrode 24 (or vibration of reed 64 toward .and away from elecan electrodel 24. The reed may be of the free- 'free type andenergized by air under pressure on one side thereof, or in .any other suitable manner. i

It will be understood that the reed 64 of Fig. 2

corresponds to the wheel l0 and its projections Such of the ele .rents of the circuit oi' Fig. 2 which are counterparts of elements of Fig. 1, have reference characters applied thereto corresponding to those Vapplied to the similar elements of the' previously described circuit .of Fig. 1.

includes a high resistance 66 connecting the reed 64 to ground through the secondary 36, the value of the resistance being such as to permit discharge oi' a condenser 58, which 15in parallel therewith, at a predetermined slow rate.

The condenser 68 is charged from a source 10 of direct current potential upon closure oi' a key operated switch 1,2, the rate of charging being controlled by series resistors 14 and 16 and a conf denser 18 which connects the junction of resistors 14 and 16 to-ground. When the switch 12 is closed, the polarizing potential on the reed 44 builds up gradually, as condensers 16 and 46 are trode 24). The resultant voltage change on the electrode 24 will be of a relatively small value, and will be amplified by the electron discharge device 46 so asto appear across the output terminals'56 at a useful amplitude.

Assuming for a. moment that the high frequency generator is not in operation, upon connect-ing the contactor 40 to a point 4| on the resistance 3U which isv at ground potential, it might be taken for granted that the signal generated by the apparatus would cease, since theoretically the wheel I0 and electrode 24 would be at the same potential. It is found, however, that this is not the case, but that under these circumstances the signal will continue at a relatively low amplitude, but at sufficient amplitude to make the generator undesirable for'purposes which require that the signal be completely cut oil upon removal of the polarizing potential. This continued generation of the signal is due to the presence of what I, lfor convenience, refer to as a residual potential. or residual polarization between the wheel I0 and electrode 24. Furthermore,.it will be found that this residual potential can, in each instance, be removed by adjusting the sliding contactar 40 to some position on the resistance 3U other than a point at ground potential. However, there is no definite potential which will always be effective to neutralize or compensate for this residual potential. Sometimes a neutralizing lor compensating potential which is positive with respect to ground, and at other times a neutralizing potential which is negative with respect to ground, may be required to cause the generator'output to drop to zero value.

When, however, the generator 42 is rendered operative to superimpose a high-frequency voltage upon the direct current polarizing potential,

it is found that the residual .potential eiIect disappears as soon as the..sliding contacter 40 is connected to a pointofv ground potential. Thus, by virtue of the superposition of the radio frequency upon the polarizing potential, the generator may be easily and reliably controlled to cause it to discontinue generating the signal Whenever desired.

As above intimated, I have been unable to determine, with any degree of certainty, the exact cause of the residual polarization of the generator. It may be caused by a minute film of oil, metallic oxide, or adsorbed gas on the wheel l0, or on its electrode 24, or on both. Or particles of dust or other foreign matter may adhere to the wheel or electrode. The effect appears to be less pronounced as the atmospheric humidity increases. If, as I believe, the effect is due to the presence of a dielectric film or dielectric particles on the wheel or electrode, it would be expected that increased humidity would render the dielectric" less elfective'to retain a residual polarizlng potential. It is within the realm of possibility that the residual polarization is due to the contact potential `between the different metals, included in the polarizing circuit, but this explanation of the phenomenon appears to be less likely to be cor- The amplitudey ofthe high frequency voltage supplied by the oscillatory generator 42 need not be very great, but only suilicient to exceed by a reasonable margin the residual potential under the most adverse conditions of operation. In practice, however, the alternating voltage supplied by the generator 42 may be relatively high, since its effect is confined to the generator circuit because the high frequency is filtered from the output circuit. v

In so far asindlcated by my experiments with this type of apparatus, the high frequency voltage supplied by the generator l2`may be im#- pressed across the capacitorelther in the circuit between the wheel (reed) and ground, or in the circuit between the electrode and ground, or may be impressed directly upon the capacitor by in-.- duction. The essence of the method of eliminating the undesired residual polarizing potential is that a high frequency alternating voltage be present lbetween the two capacitor parts at the time the polarizing potentialis removed.

'I'he generator may be used as a frequency source for electrical musical instruments or any other purpose for which a signal of determinable frequency and readily controllable amplitude may be found to be desirable.

While I have shown and described a particular embodiment of my invention, it will be apparent tol those skilled in the art that numerous modi-v results of the invention may be obtained by substantially the same or equivalent means.

I claim:

1. Inan electrical 8181181 generator, the 'combination of a'varying capacitor, means to derive movement of one of said` parts relative to thel other, a generator of high frequency oscillations, and a circuitfor impressing oscillations from a said generator across said parts, thereby to eliminate residual polarizing potential across said parts upon removal o the polarizing potential thereon.

3. A device for originating an electrical Wave, comprising a variable condenser, means to opcrate said condenser to cause periodic variations in its capacity, means operable at will to impress a direct current polarizing potential on said condenser, and means to eliminate any residual po-` larizing potential which may remain on said condenser after the `direct current polarizing potenti'al is removed, comprising a source of high fresaid condenser.

4. In an electrical signal generator, the combination of a varying capacitor having relatively moving parts separated by a dielectric, means for applying a polarizing direct current potenv tions, andl a circuit for impressing oscillations from said source across said parts, thereby to eliminate residual polarizing potentials across said parts upon disconnection ofthe polarizing potential therefrom.

5. Inan electrical signal generator, the combinationof a varying capacity generator, a signal output circuit therefor, a source of direct current, means to connect said source across said generator to energize ,the latter, a high frequency voltage source, and a circuit for continuously impressing a high frequency voltage from said latter source across said generator.

6. 'I'he method of removing the residual po-` tential from an electrostatic generator which is polarized'` by the application of a direct current potential thereto, which comprises applying a high frequency alternating voltage to the generator. and filtering vthe high frequency voltage from the output of the generator.

' 7. 'I'he method or removing the residual polarization from a varying capacity type generator which ispolarized at intervals by the application of a direct current potential thereto, which comprises. continuously applying a high frequency alternating voltage to the generator. y

' i LAURENS HAMMOND. 

